490 M. A. Claudet on the Dynactinometer. 



This is true when they operate, as they do, considerably reducing 

 the aperture of their object-glasses by very small diaphragms, 

 because in this case all the rays are nearly parallel, and the small 

 difference which can exist is imperceptible ; but let them operate 

 without diaphragms upon near objects, and they will soon find 

 that their object-glasses cannot give very distinct images without 

 having regard to the question of the photogenic focus. 



It may be clearly demonstrated, by an easy experiment well 

 knowTi in optics, that the adaptation of diaphragms before the 

 aperture of object-glasses, by intercepting oblique rays, has the 

 effect of representing objects situated in various planes with as 

 much distinctness as if they M^ere in the plane giving the mathe- 

 matical focus. 



If we make a pin-hole in a card, both short- and long-sighted 

 persons can through this hole read distinctly a book placed at 

 an equal distance from their eyes ; and they can move the book 

 backwards and forwards from the point of their vision without 

 losing the correct form of the types. This sort of diaphragm does 

 not change the focus which nature has given them ; but it ex- 

 tends it by only allowing parallel rays to proceed to the retina. 



It is so with regard to object-glasses, the chromatic aberration 

 of which is destroyed, or rather rendered imperceptible, by the 

 use of diaphragms of very small apertures. For this reason, a 

 camera obscura, one of a common description, can at once, with 

 equal distinctness, represent objects situated in near and distant 

 planes. This effect is well illustrated in those beautiful \iews 

 taken on plates or paper by all kinds of object-glasses, such 

 as those produced in Paris from the house of M. Lerebours on 

 the Pont Neuf, in which the statue of Henry IV., situated as 

 near as twenty or thirty paces from the house, is as clear and 

 distinct as the long palace of the Tuileries, all the bridges in suc- 

 cession on the Seine, and all the buildings which for a consider- 

 able length cover the two opposite quays. 



If all the visual foci can coincide for distances so considerable, 

 it is not sux'prising that the photogenic foci themselves should 

 coincide with the visual foci. Therefore, if all photographic 

 operations could be perfonned with object-glasses having their 

 apertures sufficiently reduced, the problem of the two foci would 

 be resolved, and we need not trouble ourselves about the question 

 relating to their coincidence or separation. But the case is very 

 different when we have to take portraits. For this operation we 

 require the greatest rapidity ; and as the power of object-glasses 

 is in ratio of their surface, the optician has to adopt curvatures 

 which allow the greatest apertures possible to be employed. It 

 is then that the differences between the two foci and their con- 

 stant variations are manifested in the highest degree. 



